Spirochaetes

Spirochaetes are a diverse phylum of distinctive, highly motile bacteria that are typically slender and spiral shaped. They are generally Gram-negative and are best known for their unique motility apparatus: periplasmic flagella, also called axial filaments, that run lengthwise between the inner cytoplasmic membrane and the outer membrane. The rotation of these axial filaments produces a corkscrew-like movement, enabling the cells to move through viscous media such as mucus and gel-like environments.

Morphology and physiology of spirochaetes are adapted to a range of habitats. They usually measure about 0.2 to 0.5 micrometers in diameter and can be several micrometers long, with shapes ranging from loosely to tightly coiled spirals. Growth is often slow, and many species are difficult to culture in standard laboratory conditions, requiring specialized media and environmental parameters. They occur as free-living organisms in aquatic or moist environments as well as commensals or pathogens in animals, including humans.

Taxonomy and clinical relevance. The phylum includes several notable genera such as Treponema, Borrelia, Leptospira, and Spirochaeta. Some spirochaetes are disease-causing pathogens in humans and animals. Pathogenic treponemes include Treponema pallidum, associated with syphilis and related diseases; Borrelia species cause Lyme borreliosis and relapsing fever; Leptospira species are responsible for leptospirosis. Transmission modes vary by genus, including sexual contact, vector-borne spread, and exposure to contaminated water.

Diagnosis and treatment. Laboratory diagnosis relies on serology and molecular methods, with culture requiring specialized techniques. Antimicrobial therapy, often including penicillin or doxycycline, is used to treat infections, with regimens tailored to the specific disease and stage.